1. Field of the Invention
This invention generally relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming in which cleaning and discharging operations are efficiently performed relative to an image carrying member, an intermediate transfer member, and associated members.
2. Discussion of the Background
In image forming apparatuses such as copying machines, facsimile machines, printers, etc., a large number of techniques have been introduced, relating to cleaning and discharging of members associated with an image forming operation involving usage of toner. In particular, cleaning and discharging are important in a full-color image forming apparatus which is provided with an intermediate transfer member in addition to a commonly-used image carrying member. In such a full-color image forming apparatus, primary and secondary transfer operations are in turn performed so as to transfer a plurality of mono-color-toner images separately formed on the image carrying member onto a transfer sheet at one time via the intermediate transfer member.
More specifically, the image carrying member and the intermediate transfer member are arranged to contact each other so as to perform a primary transfer operation for transferring each mono-color-toner image from the image carrying member to the intermediate transfer member. For this, the full-color image forming apparatus is provided with a charge applying member for applying a charge to the intermediate transfer member to generates an electric field which generates a force to help such primary transfer operation. After a number of times of the primary transfer operation, a plurality of mono-color-toner images are overlaid with precision as one full-color-toner image on the intermediate transfer member. Then, a secondary transfer operation is performed to transfer this full-color-toner image held on the intermediate transfer member onto a transfer sheet which is also in contact with the intermediate transfer member.
The above-described intermediate transfer member is often used in a belt shape or a drum shape. An intermediate transfer belt, for example, typically has a medium range of a volume resistivity from about 10.sup.8 .OMEGA.cm to about 10.sup.11 .OMEGA.cm, which normally does not require operations for discharging the surface of the intermediate transfer belt. This helps the cost reduction.
In using such an intermediate transfer member having a medium range of volume resistivity, the surface of the intermediate transfer member is applied with a bias to perform the primary transfer operation and thus has a charge thereon. However, this charge will leak through members in contact with the rear surface of the intermediate transfer member and no charge will therefore remain on the surface of the intermediate transfer member in a relatively short time period after the application of the charge.
As a result, the intermediate transfer member has the voltage which is 0 and greatly different from the voltage of the toner image transferred through the primary transfer operation. Due to this voltage difference, toner particles forming the toner image, particularly the topmost-laid mono-color-toner image, are attracted to the surface of the intermediate transfer member. This results in a toner dispersion in which the toner particles are dispersed on the surface of the intermediate transfer member. Such a toner dispersion may badly cause a dirty background of an image, a blur of an image such as letters, and so forth and therefore make an image deteriorated in quality.
To avoid this problem, the image forming apparatus has used the intermediate transfer member which has a high volume resistivity of about 10.sup.13 .OMEGA.cm. In using the intermediate transfer member having the high volume resistivity, the surface of the intermediate transfer member charges during the primary transfer operation due to an occurrence of discharge from the image carrying member and thus increases the voltage on the surface. Because of the high volume resistivity, the charge on the surface of the intermediate transfer member will not leak through the members in contact with the rear surface of the intermediate transfer member. Thereby, the difference of voltages between the intermediate transfer member and the toner image held on the intermediate transfer member is made relatively smaller. This helps to prevent the above-described toner dispersion.
In this case using the intermediate transfer member having the high volume resistivity, or the volume resistivity of at least 10.sup.11 .OMEGA.cm, the charge will remain on the surface of the intermediate transfer member till the time when the next primary transfer operation starts. This makes it difficult to generate the same electric field as made during the previous primary transfer operation. In this case, accordingly, the charge remaining on the surface of the intermediate transfer member need to be discharged before starting the next primary transfer operation.
In addition, when a transfer sheet is jammed during the image forming operation in the image forming apparatus, the toner image held on the intermediate transfer member may pass a region where the secondary transfer operation is conducted, without being actually transferred onto a transfer sheet. This toner image needs, of course, to be removed before the next toner image is formed on the intermediate transfer member. However, a common cleaning member such as a cleaning blade alone cannot sufficiently remove the toner because the full-color image forming apparatus uses a relatively large amount of toner during one time of the image forming process.
Conventionally, a corona charger is widely used as a non-contact-type discharging member for discharging the image carrying member and other members associated with the image forming process in an image forming apparatus. Such a non-contact type of discharging member typically generates ozone during discharging, which is undesired from the environmental aspect. In addition, the discharging member needs an application of discharging bias which is generated from an expensive high voltage AC (alternating current) power source. This increase a manufacturing cost.
In addition, the above-described intermediate transfer member having a relatively high volume resistivity changes its volume resistivity in accordance with various environmental factors such as temperature, humidity, and so forth. The intermediate transfer member also changes a charger level on the surface thereof in accordance with a number of layers of mono-color toner image. With these changes, if the discharging bias is not variable, the discharging operation may not sufficiently be performed, causing a reduction of efficiency of the primary transfer operation.
As for the cleaning in the full-color image forming apparatus, it is required a relatively high level of cleaning performance, as described above. Conventionally, this is achieved by pressing the cleaning member relative to the intermediate transfer member. However, since the intermediate transfer member is rotating, the adjustment of pressure by the cleaning member has a relatively narrow margin and therefore it cannot be adjusted in a satisfactory manner.
In addition, the above-described discharging operation is needed to be performed relative to a transfer sheet carrying member as well as the intermediate transfer member. The transfer sheet carrying member carries a transfer sheet having a toner image transferred from the intermediate transfer member through the secondary transfer operation. During the secondary transfer operation, the transfer sheet carrying member is commonly applied with a bias to help the performance of the secondary transfer operation. This bias may remain on the transfer sheet carrying member after the secondary transfer operation and interferes the generation of the electric field for the next secondary transfer operation, resulting in an inferior image quality. Such a charge problem on the transfer sheet carrying member is addressed by employing a non-contact-type discharging member which involves an ozone problem.